The Faraday effect, known since the early 19th century, names the way that the polarization of a light beam is changed as a magnetic field changes that is applied to a substrate of almost any material through which the light beam passes. The resonant Faraday effect, known since the second half of the 20th century, names the way that the polarization of a light beam is changed as a magnetic field applied to a material (through which the light beam passes) changes to a degree much greater than the degree provided by the Faraday effect alone. The "resonant" Faraday effect, the much greater change in polarization with change in magnetic field than that obtained with the Faraday effect, is exhibited in semiconductor quantum wells whenever the energy (wavelength) of the excitation light corresponds to the difference in the energies of one pair of the conduction and valence Zeeman-split subbands of the quantum wells. Various modulators and other magneto-optic devices utilizing the Faraday effect are known to those skilled in the art, and reference may be had to a publication entitled Excitonic Faraday Rotation in CdTe-Cd.sub.1-x Mn.sub.x Te Quantum Wells, by Buss et al., Solid State Communications, V. 94, No.7, (pp. 543-548, 1995), incorporated herein by reference, which suggests that a magneto-optic isolator may be implemented with semimagnetic semiconductor quantum wells utilizing the resonant Faraday effect.
The so-called quantum confined Stark effect, known in the last quarter of the 20th century, names the way the transmission (absorption) spectra of excitation light applied through a quantum well of a semiconductor material is varied with the electric potential applied thereto via tuning electrodes. As will be appreciated by those skilled in the art, the applied electric field varies the energy of the available exciton transitions and therewith the transmission spectra of the excitation light. Various optoelectronic modulators and other devices utilizing the quantum confined Stark effect are known and reference in this connection may be had to an article entitled High-Speed Optical Modulation with GaAs/GaAlAs Quantum Wells in a p-i-n Diode Structure, by Wood et al., Appl. Phys. Lett. 44 (1), (pp. 16-18, Jan. 1984), incorporated herein by reference, for a description of an absorptive optoelectronic modulator utilizing the quantum confined Stark effect.
The utilities of the heretofore known magnetooptic modulators and other devices and of the heretofore known optoelectronic modulators and other devices for optical communications and signal processing applications have been limited by the intrinsic nature of the respective classes of devices. For the magnetooptic modulators and other devices based on the Faraday and resonant Faraday effects, bandwidth limitations, imposed by the inability to quickly change the applied magnetic fields, restrict their operation frequencies to well below the mid-GHz frequencies, and beyond, called for by present day and future optical communications and signal processing applications. The magnetic field of the heretofore known magnetooptic modulators and other devices is controllably varied by varying the current through an electromagnet operatively coupled thereto, whose power requirements additionally limit the utility of the magnetooptic class of devices for signal processing and optical communications applications calling for low power.
For the heretofore known optoelectronic modulators and other devices based on the quantum confined Stark effect, modulation depth, which may be only a small percentage in a typical case, is limited by the materials-dependant absorption peaks of the exciton transitions of the quantum wells of their respective semiconductor substrates. Although attempts have been made to improve the modulation depth by building devices with multiple quantum wells, the effort has met so far with limited success. Devices having multiple quantum wells vertically stocked under the tuning electrodes to improve the modulation depth, where the excitation light is passed generally perpendicularly through the wells, however, require more voltage to operate the more wells are stocked in the device, while devices having multiple quantum wells wherein the excitation light is passed generally parallel ("horizontal") through the wells require correspondingly elongated tuning electrodes and pay the price of reduced bandwidth (slower speeds), since the more distance the excitation light traverses the greater is the required horizontal elongation of the tuning electrodes and the longer is the time constant of the electric field applied thereby.